The biological carbon pump (BCP) transfers atmospheric carbon from the surface ocean to deeper waters via particulate organic carbon (POC), playing an important role in global carbon balance and climate stability. Its strength and efficiency are primarily assessed through the downward export flux of POC, which is regulated by the production sources, particle sizes, and remineralization processes of POC. The variation in those factors across latitudes and gyres in the Northwestern Pacific Ocean (NWPO) makes it an ideal zone to study the regulation mechanisms of POC export. Size‐fractionated (1 –51 μm; >51 μm) POC and particulate hydrolysable amino acids (PHAA) were collected and analyzed from low- and mid-latitude NWPO in our study. Indexes derived from PHAA were used to trace the sources, compositions, and biodegradation of POC in both large and small particle sizes. Our findings demonstrate the joint control of POC export by particle size and microbial degradation as follows: